Fundamental frequency detector utilizing plural filters and gates



1968 G. E. PETERSON ETAL 3,364,

FUNDAMENTAL FREQUENCY DETECTOR UTILIZING PLURAL FILTERS AND GATES File A g- 2 1965 2 Sheets-Sheet l I I l Filter Ampllfler I O P I j N 1 0-64 P43 /7 T I P 1 Filter Detector I u P /8 T 0-103 /4 r I U I l Filter AmpIifier Gate T 7 /6 G rrZo gjgvgmm 0 2L 8 219072 Fla .2 (552a ipzewan 139' W ZZZ? Jan. 16, 1968 G. E. PETERSON ETAL FUNDAMENTAL FREQUENCY DETECTOR UTILIZING PLURAL FILTERS AND GATES Filed Aug. 23, 1965 2 Sheets-Sheet 2 ym w/vuw J In uf .FIG. 3 p

W 50 5/ -52 cps High cps High GOcps High Pass Filter Pass Fi I fer Pass Fi lfe l l l 544 --l67cps Low Pass Fl lfer H P 554 64cps Low Pass Filfer -lDeiecior 56A P548 j |O3cps Low Pass Fil'rer Amplifier B 7 98 cps Low Pass Fi lier Detector 56B l T54 I58 cps Low Pass Filler Amplifier Gate i *-5 fi -Hl5lcps Low Pass Flllel' De'rector 55c 60 $242 cps Low Pass Filler Amplifier Gafe L l F 55D requency -l23lcps Low Pass Filter H Deiector 7 Meter 54E -l370 cps Low Pass Filfer -l Amplifier w-l Gaie 55E 7 353 cps Low Pass Filier Detector 56E 54F i -w{565 cps Low Pass Filter Amplifier t Gafe 55F 538cps Low Pass Filler D t t 546 86l cps Low Pass Fi lier Amplifier Gale 55G L820cps Low Pass Filfer Deiecior "54H 1 ---l|3|2 cps Low Pass Filfer -=LAmplifier r 55H t250cps Low Pass Filler H Defeclor l-- 56H F55I\ f -lEOOOcps Low Pass Filter Amplifier Gate lnveniozps Gordon al eiewon Gaza QPQiQUSOZY/ America as represented'hy the fiecretary of BSTRACT OF ms DISCLOSURE This invention deals primarily with means of automatically analyzing the fundamental signal present in a signal containing fundamental and harmonic frequencies and is practically adapted to use human speech analyzers.

A plurality of gating means are provided in a plurality of filter circuits, which have consecutively increasing pass bands, to prevent the passage of any signal higher than the fundamental.

1 This invention relates to a signal analyzer and more specifically to a fundamental frequency analyzer for determining the fundamental frequency appearing in a complex electrical signal. I

Previous methods of fundamental voice frequency analysis are basically two in nature. in one method, a low pass filter with only a few decibel per-octave attenuation is employed to emphasize the fundamental frequency, and a .a frequency detector is then used to indicate the frequency of the wave at the outputof the filter. In the second method, successive major peaks in the amplitude wave are detected as a function of time-Spacingbetween these peaks. may provide a measure of the'period of the fundamental of the speech wave.

The major disadvantage of the first method described is that the fundamental frequency may be very weak in amplitude in'some' places in the speech wave. When this occurs, the second harmonic may be higher in amplitude than the fundamental at the output of the low pass filter and is thus measured by the frequency detector. A second disadvantage is that-a single filter is not applicable to'a wide range of fundamental frequencies, so that either the range of the circuit is very restricted or manual switching must be employed according to the range of the' speakers voice under analysisfllhe major disadvantage of the majorpeak may occur within one voice period in the speech wave. If two or more peaks which are approximatefirst means passes signals, and output means coupled to the first and second means for indicating signals passed; by the .first and second means.

Furtherv objects of the advantages of the invention will become apparent from the following specification, claims, and figures, wherein:

FIG. 1 is a block diagram of one invention;

FIG. 2 is a circuit diagram incorporating the layers tion; and

'FIG. 3 is a block diagram of a speech analyser.

FIG. 1, is a simplified block diagram of a signal arm lyzer which is practically adaptable for separating fundamental frequencies in, a signal which contains harmonics thereof. An input 11 is shown which is capable of rest-iv ing a signal to be analyzed. Three low pass filters 13, and 14 are shown connected to input 11 and all receive the signal to. be analyzed. Amplifiers 15 and to are coupled to filters 12, 14 and serve to increase the amplitude of signals passed by the filters. A detector circuit 17 connected to filter 13 and a gate circuit 18 coupled to amplifier 16 and circuit 17 has its output coupled to an output circuit 19. Amplifier 15 is'also coupled to the out-= put 19 which is schematically shown as a block diagram but would normally be a frequency meter or oscilloscope which serve to indicate to'receive the fundamental'frc .quency. 1

Low pass filter 12 is selected to have characteristics whereby a given pass band is obtained which allows sig nals to be passed therethrough within a'given frequency range. Filter 13 has a pass band which is less than the pass band of filter 12. Filter M's'pass band is approximately twice that of filter 13, although in one successful embodiment of the invention it is slightly smaller than twice the pass band of filter 13. 1

Assume, for the sake of illustration and not as it limits.- tion upon the invention, that filter 12 has a pass band from 0-67 cycles per second, filter 13 has a pass band of second method described above is that more than one ly equal in amplitude occur within one period, then a false reading of the fundamental voice frequency is very likely toresult.

Therefore, it is the object of this invention to provide an improved signal analyzer. I

It, is a further objectof this invention. to provide an improvedsignal analyzer for obtaining the fundamental frequency of speech signals.

- It is a further object of this invention to provide a signal analyzer which iscapable of obtaining the fundamental frequency regardless of the strength of the overtones present in the signal.

it is yet a further object of this invention to provide a signal analyzer comprising, an input means for receiving a signal to be analyzed, first means, for passing a first selected-band of signals coupled to the input means, second means for passing a secondselected band of signals including first band of signals coupled to the input, third means coupled to the input means passing a third selected band ofsignals and coupled to the, second means for preventing the. second means from passing signals when the 0-64 cycles per second and filter 14 has a pass band of 0-103 cycles per second. When a signal is rceived which has a fundamental frequency of 50 cycles per 'secondanrl includes a harmonic of 100 cycles per second then filters 12 and 13 will pass the fundamental frequency and filter 14 will pass both the fundamental and the harmonic. Amplifier 15 produces a signal which is passed to output circuit 19. Detector 17 generates a signal in response to the fundamental signal passing filter 13 and serves to render gate 118 closed to signals generated by amplifier 16, and both the fundamental and haromnic signs is passed by'filter 14 are blocked from entering output c.' rcuit 19. Thus, although the signal received at input 11 is composed of two signals, that is a fundamental and a har monic, only the fundamental appears in the output i .If the composite signal received is cycles per second and includes a harmonic at 140 cycles per second then filters 12 and 13 do not pass any substantial amount of the signals. Filter 14 passes the 70 cycle signal thru to amplifier 16 and gate 18, normally open in absence of a control signal from detector 17 allows the 70 cycle sig nal to pass to output circuit 19.

FIG. 2 shows a detailed circuit diagram for amplifier 16, detector 17 and gate 18 of FIG. 1. Assuming the first example of received signal as set forth above, filters 13 and 14 pass the signal as described, the 50 cycle signal as represented by sine wave is received across a pair of terminals 25, 26 and is amplified through transistor circuit 27 producing a signal 28 as set forth at the collector of transistor 27. This signal is coupled through a'capacitor 29 and a pair of diodes 30, 31 and produce in conjunction with a capacitor 32 a signal as shown at point 33. The composite signal of S0 and cycles per second is received from filter 14 at a pair of terminals 34,-

embodirnent the 35 and would be amplified by transistor 36. However, the signal 33 serves to bias a pair of transistors 31, 38 to provide a low impedance path to any alternating current signal appearing at point 39 and a transistor 40 coupled thereto through a resistor 41 and capacitor 42 will produce ze'ro signal out across a pair of terminals 43 and 44. Thus, the output circuit 19 contains only the fundamental signal passed by the previous stage.

FIG. 3 shows an embodiment of the invention wherein three high pass filters 50, 51 and 52 are coupled thru a switching circuit 53 to the input 11. The purpose of these filters is to eliminate certain undesired frequencies which might appear in the signal but would normally not occur in voice signal analysis. A plurality of signal circuits, in cluding low pass filters, are shown designed by 54A through 541, the filter pass band characteristics are indicated in the blocks and it is to be noted that each successive signal circuit has a pass band slightly less than twice that of the previous signal circuit. That is 54A has a pass band of 67 cycles per second and the pass band of 548 is 103 cycles per second. There is further included a group of control circuits designated 55A through 55H, individually coupled between filters 50, 51, 52 and a series of gate circuits 56A through 56H. The pass band of the individual control circuit filters is slightly below that of the previous signal circuit filter to insure proper elimination of signals through the operation of the circuits. It is to be noted that with ideal filter characteristics, that is, sharp cut-off at a specific frequency it would be possible to have the signal circuit filters approximately an octave apart, however, to insure proper discrimination between signals it has been found desirable to utilize a factor of approximately of an octave increase for each successive signal circuit. a

In operation of the embodiment shown in FIG. 3 switch .53 is set to eliminate signals received below a given frequency. This is found desirable because speech signals do not normally include frequencies in the designated ranges of filters 50, 51, 52 and the noise level is thus reduced.

pass the 640 cycle signal and 54H, 541 passthe 1280 cycle signal. Gate 56A remains open allowing the 80 cycle signal to be received at a frequency meter 60. Cirsuit 553 generates a signal actuating gate 568 to pre vent any signals passing through circuit 548 from reaching the meter 60. Similarly gates 56C through 56H are activated to prevent any of the harmonics from appearing at meter 60.

i We claim: 1. An improved signal analyzer for (ltlfl'miuing the fundamental frequency in a received signal, comprising:

(a) an input circuit coupled to receive and pass received signal and having a plurality of outputs;

(b) a first circuit having a pass band from a fire pltw determined minimum frequency to a first pin mined maximum frequency coupled to an outer: said input circuit;

(0) a second circuit having a pass band from said iiiilt' predetermined minimum frequency to a second determined maximum frequency coupled to an out put of said input circuit;

(d) a third circuit having a pass band from said first predetermined minimum frequency to a third predetermined maximum frequency coupled to an output of said input circuit;

(e) a fourth circuit coupled to said second ihirz'l circuits; and

(f) an indicating output circuit coupled to receive sis nals from said first circuit and said fourth cin it, said third predetermined frequency being higher than said first and second predetermined maximum ire quencies, said fourth circuit being capable of pin venting signals from being transmitted by said tl'iirrl circuit when signals are received at said first and second circuits which are passable thereby to said. output circuit and said fourth circuit transmitting signals passed by said third circuit when the fun damcntal frequency of the received signal is higher than said first and second maximum predtilcrmitlcati frequencies.

2. The analyzer of claim 1 wherein said first ,pretie et" mined maximum frequency and said second predeter 35 mined maximum frequency are substantially equal.

3. The analyzer of claim 2 wherein said second pro determined maximum frequency is a value belt-men one and two times that of said first predetermined maximun't frequency. I

4. The analyzer of claim 3 wherein said predetermined minimum frequency is equal to zero.

5. The analyzer of claim 1 wherein said input circuit has selective means for eliminating fundamental fre quencies below a given value.

I References Cited UNITED STATES PATENTS 2,699,464 1/1955 Di Toro et al. 324 m 2,999,205 9/1961 Sichalt etal. 311' 3,215,934 11/1965 Sallen .s 3724- RUDOLPH V. ROLINEC, Primary Examiner. WALTER L. CARLSON, Examiner.

PAUL F. WILLE, Assistant Examiner. 

